As the word itself implies, “cryoablation” involves the ablation of tissue (i.e. tissue necrosis or destruction) using extremely low (i.e. cryogenic) temperatures. Typically, cryoablation requires lowering the temperature of the tissue to below approximately minus twenty degrees Centigrade (−20° C.). However, more efficient ablation procedures often call for temperatures as low as minus eighty eight degrees Centigrade (−88° C.) or lower.
In some cases, cell survivability depends not only on the cryoablation temperature, but also the rate at which the cells are cooled to the cryoablation temperature and the rate at which the cooled cells are subsequently warmed. Thus, it is often desirable to control both the cooling and warming rates in a cryoablation procedure. This control, in turn, requires that the temperature of an operative contact surface in the cryoablation device be controlled over a selected temperature range. Another instance in which it is desirable to control the temperature of a cryoablation tip or contact surface occurs when it is necessary to cryoablate tissue to a specific thickness (i.e. at a pre-selected depth from the contact surface). In such a case, it is important to control both the tip temperature and contact time to control the ablation depth.
It is often desirable to cryoablate internal tissue in a relatively non-invasive procedure. For this purpose, cryocatheters have been developed, such as the cryocatheter and associated refrigeration system that is disclosed in co-pending U.S. patent application Ser. No. 10/243,997, entitled “A Refrigeration Source for a Cryoablation Catheter.” Co-pending U.S. application Ser. No. 10/243,997 was filed on Sep. 12, 2002, is assigned to the same assignee as the present invention, and is hereby incorporated by reference herein. In one exemplary application of a cryocatheter, conduction blocks can be created in the tissue that are particularly effective for curing heart arrhythmias, such as atrial fibrillation.
In a typical cryocatheter procedure, the distal portion (i.e. cryotip) of the catheter is positioned near or in contact with the tissue requiring ablation (i.e. the target tissue). Next, the cryotip is cooled to a cryogenic temperature to thereby cool and ablate the target tissue. Typically, this is accomplished by expanding a fluid refrigerant into an expansion chamber near the catheter tip and exhausting the expanded refrigerant from the chamber through a return line. For this expansion, the pressure of the refrigerant as it enters the chamber, as well as the pressure in the return line (back pressure), will affect the temperature of the cryotip and the instantaneous cooling power of the cryocatheter. In addition, for a cryoablation system in which the refrigerant undergoes a phase change during expansion (i.e. transitions from a liquid to a gaseous state), the back pressure effects the actual refrigerant boiling temperature. This boiling temperature, in turn, controls the temperature of the cryoablation catheter tip.
In light of the above, it is an object of the present invention to provide systems and methods for controlling the tip temperature of a cryoablation catheter. It is another object of the present invention to provide a temperature control system for a cryoablation device that can either stabilize the cryoablation tip at a constant tip temperature or vary the temperature of the cryoablation tip in accordance with a predetermined schedule. Yet another object of the present invention is to provide a temperature control system for a cryoablation device which is easy to use, relatively simple to implement, and comparatively cost effective.